You are limited to specific files/formats, depending on which HDTV you are working with. My advice? Simply get a $50-100 media streamer, and access your files on the network that way. Much less of a headache, and you can now play almost any file/format vs a HDTV where this is mostly marketing hype and an afterthought. Files like MKV require more than most HDTV manufacturers are willing to shell out for this feature included in their firmware. Oh, and DLNA stinks, regardless of what anyone tells you on the interwebs.
FWIW, I use a WDTV live, which will play back almost anything. Only a couple of weirdly subtitled/downloaded files wouldn't play back sound or video on a couple occasions. The interface is somewhat dated however. A prettier and nicer user experience would be jailbreaking an Apple TV, then loading PLEX on it ;).
I have a home network, and my server has many downloaded movies in .avi, .mkv, and .mp4 formats. Can the new smart TVs access my network and play these movies?